A. Field of the Invention
The present invention relates generally to photonic crystals, and, more particularly to a process for making photonic crystal circuits using an electron beam and ultraviolet lithography combination.
B. Description of the Related Art
During the last decade photonic crystals (also known as photonic band gap or PBG materials) have risen from a relatively obscure technology to a prominent field of research. In large part this is due to their unique ability to control, or redirect, the propagation of light. E. Yablonovich, “Inhibited spontaneous emission in solid-state physics and electronics,” Physical Review Letters, vol. 58, pp. 2059-2062 (May 1987), and S. John, “Strong localization of photons in certain disordered dielectric superlattices,” Physical Review Letters, vol. 58, pp. 2486-2489 (June 1987), initially proposed the idea that a periodic dielectric structure can possess the property of a band gap for certain frequencies in the electromagnetic spectra, in much the same way as an electronic band gap exists in semiconductor materials. This property affords photonic crystals with a unique ability to guide and filter light as it propagates within it. In this way, photonic crystals have been used to improve the overall performance of many opto-electronic devices.
The concept of a photonic band gap material is as follows. In direct conceptual analogy to an electronic band gap in a semiconductor material, which excludes electrical carriers having stationary energy states within the band gap, a photonic band gap in a dielectric medium excludes stationary photonic energy states (i.e., electromagnetic radiation having some discrete wavelength or range of wavelengths) within a certain energy range or corresponding frequency range. In semiconductors, the electronic band gap is a consequence of having a periodic atomic structure which interacts with an electron behaving quantum-mechanically as a wave. This interaction gives rise to a forbidden range of energy levels, the so called electronic band gap. By analogy, the photonic band gap results if one has a periodically structured material, where the periodicity is of a distance suitable to interact with an electromagnetic wave of some characteristic wavelength, in such a way as to create a band of frequencies that are forbidden to exist within the material, the so called photonic band gap.
An envisioned use of these materials is the optical analog to semiconductor behavior, in which a photonic band gap material, or a plurality of such materials acting in concert, can be made to interact with and control light wave propagation in a manner analogous to the way that semiconductor materials can be made to interact with and control the flow of electrically charged particles, i.e., electricity, in both analog and digital electronic applications.
Planar photonic crystal circuits such as splitters, high Q-microcavities, and multi-channel drop/add filters have been investigated both theoretically and experimentally in both two- and three-dimensional photonic crystal structures. For two-dimensional photonic crystal structures, the photonic crystal consists of either an array of low index cylinders surrounded by a background material of sufficiently higher index or, an array of high index cylinders surrounded by a background material of sufficiently lower index. In both cases, in-plane confinement is achieved through multiple Bragg reflections that occur due to the presence of the material lattice, which represents the photonic crystal. For some three-dimensional photonic crystal structures, namely those that consist of a two-dimensional structure, or lattice, that are finite in height, confinement in the vertical direction is achieved through total internal reflection (TIR). In either case the main limiting factor in the wide spread use of these devices is the ability to get light into and out of these structures. For this reason, optical coupling structures have a pronounced impact on the operation of any photonic integrated circuit (“PIC”).
In addition, whereas the lithography methods developed in the microelectronic industry have been successfully applied to fabricating small photonic crystal circuits, these methods are far from optimum for this purpose. For example, patterning a square centimeter of photonic crystal circuit for the near infrared wavelength requires exposing approximately 500 million circles. While not impossible, this is certainly strenuous for current lithography tools. Moreover, while slight variation in feature size is acceptable for electronic circuits, the tolerances in case of photonic crystal circuits are far more stringent for even small variation of hole sizes may compromise the performance of photonic crystal circuits.
Thus, there is a need in the art for an improved process for making photonic crystal circuits.